Charged particle focusing systems have many valuable uses, including those wherein the charged particles are accelerated. For example, they are used: in microprobe systems to measure the concentration of elements in a sample; to provide one or two dimensional composition and structure maps using: PIXE (Particle Induced X-ray Emission), in STIM (Scanning Transmission Ion Microscopy); for tissue analysis of biological samples etc. High energy focused ion beam (HEFIB) microprobes have developed into very powerful analytical tools in recent years. The lateral resolution of microprobes is improved toward the submicron regime requiring high-quality probe-forming systems. Like all charged particle accelerator system, the HEFIB system is composed of certain basic components. Such basic components include: (i) a source of charge particles; (ii) a means for accelerating the charged particles; (iii) a tube pumped to a partial vacuum in which the beam of accelerated charged particles travel; (iv) a lens system for focusing the charged particle beam; and (v) a target chamber.
The focusing systems for many charged particle systems are comprised of multiple quadrupole lenses, each lens having two electrostatic or magnetic poles placed in parallel with the optical axis. A strong lens effect can be obtained owing to the fact that the main component of the electrostatic or magnetic field acts perpendicularly to the beam axis of charged particles. In its single stage configuration, however, the quadrupole lens is extremely asymmetrical, because it acts as a convergent (convex) lens in the XZ-plane and as a divergent (concave) lens in the YZ-plane. Therefore, when the quadrupole lens is used for beam convergence in an ion microprobe mass analyzer, electron and ion beam lithography system, Auger electron spectroscopy system, x-ray microanalysis system or the like, it has to be combined with like lenses in two, three or more stages. The great majority of lens systems for MeV energy systems is accomplished by quadrupole lenses, the great majority of which employ various combinations of magnetic quadrupole lenses, including doublet, triplet, quadruplet and quintuplet. The most popular quadrupole systems are the Russian Quadruplet and the Oxford Triplet. The Russian Quadrupole configuration is popular because of its symmetry and its orthomorphic character that permits the use of circular object diaphragms. The Russian Quadruplet consists of a set of four quadrupoles (magnetic or electrostatic), which alternating polarities. The two outer ones are coupled together, with the length l1 and the excitation K1=K4, as are the central ones, which the length l2 and the excitations K2=K3. The separation between the first and the second lenses and the third and the fourth is s1, and the separation between the middle lenses is s2.
The characteristics and throughput of the aforementioned charged particle beam apparatuses improve in proportion as the beam current value rises when the beam is converged to a minute diameter. Insofar as there are no problems regarding the source size and radiation current, the beam current can be increased solely by enlarging the effective beam converging angle. The size of the beam converging angle is, however, limited by the aperture (spherical) aberration of the lens. For optimum spatial resolution a small beam-spot is required, but because of aberrations, which are present to some extent in all designs of probe-forming systems, this can typically only be achieved at the expense of a reduction in beam current.
Thus, one of critical requirements for the components of a microbeam facility is low aberration/high demagnification probe-forming systems. While a variety of conventional quadrupole configurations exist, including quadrupole doublets, triplets, and quadruplets all suffer from the same drawback of not being able to adjust the focal length once the lens system is set in place within the beam line apparatus.
Therefore, there exist a need in the art for a quadruple lens system that will allow for the adjustment of focal length after the lens system is set in place in order to adjust the focusing system to mitigate intrinsic aberration of the lenses to achieve a optimum small beam spot.